Typically, digital image compression techniques are characterized as lossless or lossy. When an image's data file is losslessly compressed, its subsequent decompression produces a data file that is identical to the original. In lossy compression, the original and the decompressed file differ, but the differences may be acceptable if the essence or perceived visual quality of the decompressed image file is retained. When compressing a data file, a target compression ratio is often sought in order to produce a compressed file of manageable size. The compression ratio is the ratio of the size of the compressed data file before compression to the size of the data file after compression.
Data compression techniques are frequently used to compress either digital continuous tone images or binary images. Continuous tone images are images that have a virtually unlimited range of color or shades of gray. Binary images are black and white images that lack any shades of gray. Binary images use one bit to represent an image pixel. A pixel, also known as a picture element, is generally understood to be the smallest unit of an image that a particular image generation device can produce, store, or transmit. Many image output devices, such as digital printers, monochrome facsimile devices or raster image processors represent images in binary format.
The concept of introducing loss in continuous tone color images is well documented in the literature. One well-known example is the image compression standard for continuous tone still images developed by JPEG (Joint Photographic Experts Group). Another example is the Wavelets algorithm. These, and others known techniques, define lossy image compression approaches that can be “tuned” to provide a wide range of tradeoffs between image quality and compression ratio.
For binary images, various well-known compression approaches exist such as those defined by CCITT, Lempel-Ziv and Deflate. However, these lossless methods often cannot provide adequate compression ratios for meeting the real-time bandwidths required to drive many high-speed printers and other electronic devices.